1. Field of the Invention
The present invention relates to a light emitting device, especially, a light emitting device and an electronic equipment having an organic light emitting diode (OLED) formed on a plastic substrate. Further, the present invention relates to an OLED module wherein ICs including a controller and the like are mounted on the OLED panel. In this specification, the light emitting device is a general term for the OLED panel and the OLED module.
2. Description of the Related Art
In recent years, a technology constituting a thin film transistor (TFT) using a semiconductor thin film (in the range from about a few to a few hundreds nm in thickness) formed on the substrate having an insulating surface has drawn attention. A thin film transistor is widely applied to electronic devices such as an IC, an electro-optic device or the like, and particularly, there is an urgent need to be developed as a switching element for an image display device.
Although as for applications utilizing such an image display device, a variety of applications are expected, particularly, its utilization for portable apparatuses has drawn the attention. At present, although many glass substrates and quartz substrates are utilized, there are defaults of being easily cracked and heavy. Moreover, the glass substrates and quartz substrates are difficult to be made larger in terms of conducting a mass-production, and these are not suitable for that. Therefore, the attempt that a TFT element is formed on a substrate having flexibility, representatively, on a flexible plastic film has been performed.
However, since the heat resistance of a plastic film is low, it cannot help lowering the highest temperature of the process. As a result, at present, a TFT is formed which has not so excellent electric characteristics compared with those formed on the glass substrates. Therefore, a light emitting element having a high performance by utilizing a plastic film have not been realized yet.
In these years, research of an active matrix type light emitting device (hereinafter, simply referred to as a light emitting device) having a light emitting diode as a self-luminescence type element is intensified. The light emitting device is also called as an organic EL display (OELD) or an organic light emitting diode (OLED).
The OLED has high visibility since it emits light for itself and does not need a backlight which is necessary in a liquid crystal display (LCD), and it is optimum to be made thinner, and there is no limitation about a visual field angle. Therefore, a light emitting device using the OLED is noticed as a display device taking the place of CRTs and LCDs.
In case that it becomes possible to make a light emitting device in which an organic light emitting element is formed on a substrate having flexibility such as plastic film, it is thin in thickness and of light weight and can be used for a display having a curved surface and a show window. Therefore, its application is not limited only to portable apparatuses but it has a broader range of applications.
However, a substrate comprising a plastic is generally easy to transmit moisture and oxygen through it, and deterioration of an organic light emitting layer is expedited by these staffs, and therefore, a light emitting device is particularly easy to be short-lived. Thus, in the related art, an insulating film which comprises silicon nitride and silicon oxynitride is disposed between a plastic substrate and an OLED so that mixture of moisture and oxygen in the organic light emitting layer is prevented. However, in the insulating film which comprises silicon nitride and silicon nitride oxide, it is hard to adequately prevent moisture and oxygen from being mixed in the organic light emitting layer.
In addition, a substrate such as a plastic film is generally weak against heat, and in case that temperature for forming an insulating film such as silicon nitride and silicon oxynitride is raised too much, the substrate is made to be easily transformed. Further, in case that film forming temperature is too low, film characteristic is deteriorated so that it becomes hard to adequately prevent moisture and oxygen from being mixed.
Further, in case that driven is an element which is disposed on the substrate such as the plastic film, it becomes an issue that heat is developed locally so that a part of the substrate is transformed and degenerated.
Furthermore, in case that thickness of the insulating film such as silicon nitride and silicon oxynitride is increased in order to prevent moisture and oxygen from being mixed, stress is enlarged so that it becomes easy to suffer some cracks. Moreover, in case that film thickness is increased, the film is apt to suffer some cracks when the substrate is bent. Further, when the substrate is peeled off, a layer to be peeled off is bent and the layer to be peeled off suffers some cracks.
Further, in case of a TFT, when impurities such as an alkaline metal (Li, Cs, Na etc.) and an alkaline earth metal (Ca, Mg etc.) and other metal elements are diffused in an active layer in addition to moisture and oxygen, characteristic is apt to be changed.
Furthermore, even after final products are made, in case that other impurities, for example, human sweat and impurities from connecting components, are diffused and mixed in the light emitting layer and the active layer of TFT, there is a possibility that degeneration and deterioration are expedited.